waf: build PIEs if supported by the compiler

waf: build PIEs if supported by the compiler

Currently waf performs a mandatory check for compiler PIE support,
unless --without-pie is specified.
This change makes Waf only perform the mandatory check if --with-pie is
specified. If neither --with-pie nor --without-pie are specified, then
PIEs are only built if compiler support is available.

Signed-off-by: David Disseldorp <ddiss@samba.org>
The last 3 patches address bug #9910 - PIE builds not supported.

Autobuild-User(v4-0-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-0-test): Thu May 30 11:37:47 CEST 2013 on sn-devel-104